1. Field of the Invention
The present invention relates to a system for enabling medical procedures to be performed by ultrasonic heating and more particularly to a system for enabling selective heating of tissue guided by medical imaging.
2. Description of Related Art
Conventional medical imaging provides the radiologist with internal views of a patient's anatomy. Magnetic resonance (MR) imaging provides excellent contrast between different tissues and is useful in planning surgical procedures. A tumor in a patient is much more visible in an MR image than as seen in actual surgery because the tumor and normal tissue often look similar in surgery. The tumor can also be obscured by blood during surgery.
Tumors have been selectively destroyed in cancer patients using focussed ultrasound heating at the University of Arizona, as reported by B. E. Billard, K. Hynynen and Robert. B. Roemer "Effects of Physical Parameters on High Temperature Ultrasound Hyperthermia" Ultrasound in Med. & Biol. Vol. 16, No. 4, pp. 409-420, 1990 and hereby incorporated by reference. The patient is first scanned in an MRI system to locate the tumor and plan a safe trajectory between the entry and target points. A view of the heated region is provided with the use of MR temperature sensitive pulse sequences. Known MR temperature sensitive pulse sequences are described in U.S. Pat. No. 4,914,608 "In-vivo Method for Determining and Imaging Temperature of an Object/Subject from Diffusion Coefficients Obtained by Nuclear Magnetic Resonance" by Denis LeBihan, Jose Delannoy, and Ronald L. Levin issued Apr. 3, 1990. Experiments on animals show that a heated zone above a critical temperature destroys tissue. This zone increases in size with time as the heat is applied to reach a steady state or both temperature and heat flow. If the maximum temperature is limited to 100 deg. C., then the heated zone, the area exceeding a critical temperature causing destruction of tissue, approaches 1 centimeter in diameter. It is difficult to predict the heated zone geometry because the heat flow depends on the profusion of blood as well as the tissue thermal properties.
However, it is difficult to determine the location of the energy focal point without activating the energy transducer.
Currently there is a need for a method of selectively destroying tissue non-invasively without affecting adjacent healthy tissue.